Acoustic well logging apparatus



NQV. 21, 1950 c, A

ACOUSTIC WELL LOGGING APPARATUS Filed July-26, 1947 FIG. 6;

' ,INVENTOR.

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asconome GALVANOMETER FIG.

AGENT Patented Nov. 21, 1950 ACOUSTIC WELL LOGGING APPARATUS Clare H. Kean, Houston, Tex., assignor, by mesne assignments, to Standard Oil Development Company, Elizabeth, N. J., a corporation of Delaware Application July 26, 1947, Serial No. 763,940

6 Claims. 1

The present invention is directed to an apparatus for the acoustical logging of boreholes in the earth and involves the production of cylindrically symmetric waves or the like within the boreholes being logged.

This application is a continuation-in-part of my co-pending application, Serial No. 597,408, filed June 4, 1945,'in which a method of logging boreholes is disclosed and claimed. Various features of the structure shown but not claimed herein are claimed in my co-pending applications Serial No. 776,549, filed September 27, 1947, now Patent No. 2,497,172, and Serial No. 786,753, filed November 18, 1947, now Patent No. 2,490,273.

It is known to log various earth formations penetrated by a borehole by moving through the borehole an acoustic wave generator or transducer which may be excited by electrical means to cause it to vibrate and emit sound waves, and to log variations in character of the earth formations by recording variations in the acoustic impedance thereof as shown by changes in electrical impedance of the sound wave generator. It is also known to log variations inthe character of earth formations by generating acoustic waves in a borehole and measuring'the seismic velocity, that is, the speed at which elastic waves travel, in the various formations.

To be useful in commercialpractice, any logging system must be capable of reproducing the log of each borehole irrespective of the dip or strike of the subsurface formations. I have found that one desirable factor in obtaining reproducible acoustic impedance logs-is that the acoustic wave generator must be omni-directional, i. e. must send out and receive acoustic waves substantially uniformly in all radial directions with respect to the axis of the borehole.

Also to be useful in commercial practice, any acoustic impedance logging system must reflect accurately'the acoustic impedance of the earth formations surrounding the borehole and must be insensitive to the acoustic impedance of fluid, usually drilling mud or water, in the borehole.

Prior workers in the art have heretofore employed relatively high frequency, vibrating diaphragms in their wave generators. These diaphragms have been not only directional, but also have tended to set up objectionable standing waves which have made difficult, if not impos sible, the reproduction of acoustic impedance logs obtained with such systems. Furthermore, the frequencies employed by the prior workers have tended to obscure the true acoustic impedance of the earth formations because at these relatively high frequencies, the well fluid in which the logs were run has had an appreciable effect upon the logs.

I have now invented an improved method and apparatus for acoustic impedance well logging which provides accurate and reproducible acoustic impedance logs in mud or other liquid-filled boreholes. In accordance with my invention, I generate in the liquid-filled borehole acoustic waves of a selected frequency of such value that the wavelength thereof in the borehole liquid is long compared with the diameter of the borehole and is, more importantly, very long compared with .the length of the transmission path through the liquid filling the annular space between the acoustic wave generator and the walls of the borehole. By employing these long wavelengths, the efiect of the acoustic impedance of the mud upon the wave generator is minimized or practically eliminated. Further, in accordance with my-invention, I cause these acoustic waves to be radiated predominantly and simultaneously in all radial directions with respect to the axis of the borehole. By concentrating the acoustic energy pre dominantly in all radial directions the transmission path is shortened, thereby further minimizing the influence of the borehole liquid. Also by radiating the energy simultaneously in all radial directions, the efiects of irregularities in the borehole diameter and the sway and rotation of the wave generator upon its suspending cable are eliminated.

Still further in accordance with my invention, I generate cylindrically symmetric acoustic waves, i. e. waves which may be described as cylindrically expanding waves having constant phase throughout all parts of the wave front, simultaneously over a substantially cylindrical area whose length is greater than the wavelength of the acoustic waves in the borehole liquid and is many times the diameter of the borehole. By generating these cylindrically symmetric waves and radiating them throughout an area whose length is greater than the wavelength of the acoustic waves, formation of unwanted standing waves, as for example by longitudinal reflection of wave energy from top and bottom surfaces of the liquid in the borehole or by reflection from irregularities in the walls of the borehole, is substantially eliminated. 7

It is an object of the present invention to provide apparatus for logging boreholes an essential element of which is an elongated body adapted to be passed along the borehole, together with means for exciting the body to cause at least a portion thereof to vibrate in a radial direction and send out acoustical waves of greater wavelength than the diameter of the borehole which waves, to a large extent, are reflected back onto the body to damp or reduce the normal amplitude of the Vibrations.

Another: object of the present invention is to provide apparatus; for logging a borehole by pro-- ducing vibrations in a radial direction over an area of the borehole walls which is longer than a wavelength of the waves in the borehole liquid, the apparatus being so designed that measurements can be made which vary asa function of the character of the formations through which the hole extends.

It is another object. of the present invention to provide an apparatus for logging wells, an essential element of which is an elongated body defining a cylindrical outer surface, together with means to excite said body in extensional mode so that it vibrates with a major portion thereof moving inv a radial direction to send out acoustical waves which are, to a large extent, reflected back upon it and which tend to damp its vibrations, and. with means to. measure the energy required to. drive the elongated body so as to indicate: the extent of this damping action.

- Other. objects and, advantages of the present invention may-be seen from a reading of the following: description taken with the drawing in which Fig, l is an elevation, partly in section, illustrating' one embodiment of the present invention.

Fig. 2; is a. fragmentary view of another embodiiment of: the present invention which may be substituted for the elongated tubular member and driving means of Fig.1;

Fig. 3 is a fragmentary view showing another embodiment which may be substituted for the tubular member,- and driving means of the device 05E Fig: l;

Fig. 4a is in the. form of a circular diagram showing, the electrical circuit of another embodiment of the present invention;

Fig.. 5. is. an elevation, partly in section, showing a driving means suitable for use with the circuit of Figs 4';

Fig. 6 is an elevation, partly in section, of still another embodiment of the present invention employing the: circuit of. Fig. 4 and an improved means; tor mounting the elongated tubular memher; and

Fig. '7 is a sectional view showing in greater detail. the mounting means employed in the embodiment of Fig. 6.

Turning now specifically to the drawing and first to the embodiment of Fig. 1, the apparatus may be described as involving a transducer or instrument A arranged for suspension in thebcrehole on a cable B and adapted for longitudinal movement along the axis of the borehole. The cable contains insulated conductors which connect the instrument A with a power supply and recording means C adapted to be arranged at the surface of the earth. It will be understood that cable B may be thousands of feet in length and may be arranged on a hoist for the convenient lowering and raising of instrument A along the axis of the borehole but such hoisting mechanism is conventional to the art and for the purpose of simplifying the description of the present invention it has not been shown in the draw- The instrument A includes a vibrating system capable of producing a cylindrically symmetric wave. and is electrically connected with the power metric wave.

supply and recording means C at the surface of the earth so that a value which is a function of the energy required to produce the cylindrically symmetric wave may be displayed by the indicating portion of apparatus C and employed as a log of the borehole. Such a log records data from which inferences may be: drawn as to the nature of the various formations along the borehole.

Instrument A is preferably constructed with an outer shell which may be a relatively thinwalled, tubular member I2 with conically shaped closures i3 and It. at its upper and lower ends, respectively. A small passage I6, closed with cap screws H, is provided in member l3 and similarly a passage E8, closed with cap screw I9, is provided in end member It.

The vibrating system within the outer shell includes a tubular body II. It will be seen that the body II is substantially longer than the diameter of the borehole. One purpose. in using such. an elongated. body is to send. out a large amount of energy radially and toobtain. sumcient. reflected energy from the formations to dampen or lower its normal amplitude of vibration when it is caused to send out acoustical waves- Another purpose in using such an elongated body is to direct acoustic wave energy substantially perpendicularly to the axis of the borehole toward an. area of the walls thereof whose length is longer than av wavelength of the acoustic waves in the borehole liquid. Ihe body H. is constructed or a highly magnetostricti-ve material, such as nickel, cobalt or alloys thereof, and is mounted with its axis coincident with the axis or member E2. The means for vibrating member il includes a spool shaped member 22 on which ismounted a coil 23 electromechanically coupled to member H... Thev ends of the coil are connected through conductors 24 to the insulated conductors 25' and 25' of cable B.

Elongated magnetostrictive member H is mounted within the outer shell so that it is free tovibrate and produce a cylindrically sym- The mounting includes rod lilwhich. has its upper end connected to member is and its lower end. connected by a transversely extending pin I5 to a central portion of. member H and to spool 23. It will be seen that suitable holes are: provided in. tubular member H. and spool. shaped member 22v to receive pin member [-5. An additional mounting means is provided by a series of adjustingscrews 20 and 2! adiacent. the medial portion of tubular member H and. arranged between tubular shell l2 and member H.

In the mounting, of tubular member i i, it would be theoretically desirable tov employ only the rod HI- and cross pin so that the remainder would be freeto vibrate; However, the weight and space limitations embodied in the constructionof a well logging device would not allow such a mounting means to be sufficiently rigid to insure against the contact. of the ends of member H with tubular shell l2 and for this reason the series oi adjusting screws 20 and 2| are provided. It is to be understood that adiusting screws 20 and Zldo not hold the medial portion of tubular member II in an absolute fixed, position but allow it. to vibrate in a radial direction.

IEubul-ar member I I is driven or excited by a1- ternating current passing, through coil 23. Upon excitation, tubular member ll vibrates in extensional mode, that is, the ends of member H- move in a longitudinal direction. longitudinal motionproduces aradial motion or vibration such that the medial portion expands and contracts radially. The ratio of the maximum amplitudes of these motions is dependent upon the Poisson ratio of the material of the tube. The magnitude of the longitudinal vibrations of the end portions is greater than the magnitude of the radial vibrations of the cylindrical surface, but since the cylindrical area is very much greater than the area of the ends the major portion of energy withdrawn from the vibrating system is in the form of cylindrically symmetric waves in the radial direction.

It is conventional for boreholes to be drilled by the rotary method and to use drilling mud in producing the hole; accordingly, most holes logged will be filled with drilling mud. The apparatus of the present invention is-intended to be used in such mud-filled holes. While useful results may be obtained by moving element I! and coil 23 through the borehole, tubular shell i2 is preferably provided in order to protect element H from direct contact with the walls of the borehole and prevent fouling of the element H and the coil 23 with thedrilling mud. In order to prevent the entry of mud within the vibrating system and assist in transferring the desired vibrations from element II to the walls of shell [2 and thence through the mud to the walls of the borehole, the chamber defined by tubular member l2 and ends I3 and I4 is completely filled with a liquid, such as Water. Other suitable liquids having similar density and freedom from changes in viscosity with changes in temperature may be employed. Passages l6 and I8 are provided to allow the convenient filling and draining of liquid from the chamber. It is not necessary to seal the chamber defined by members l2, l3 and E4 in a pressure-tight manner inasmuch as a small amount of liquid leakage into or out of the chamber as the device is being used will not ordinarily interfere with this operation. However, care should be taken to remove completely all gas bubbles likely to be entrapped in the chamber and to prevent entry of gas into the chamber if erratic results are to be avoided. This can be facilitated by flushing the air out with carbon dioxide, which, being soluble in water, will not form free bubbles. The power supply means and indicating means C are adapted to be placed at the surface of the earth and include a means 39 for supplying alternating current, a means 3i, such as a battery, for supplying direct current, a tuned detector 32, a recordin means 38 and a bridge 33. The means 3i for supplying direct current is connected to conductors 25 and 25' of cable B by conductors 35 and 36, respectively. Conductor 36 has a choke 31 arranged therein.

The bridge 33 includes apices 40, 4!, 42 and 43. The means 30 for supplying alternating current is connected to apices 40 and. 4| by conductors 44 and 45, respectively. A tuned detector 32 is connected across apices 42 and 43 by conductors 43 and 41, respectively. A recording galvanometer 38 is connected to tuned detector 32 by conductors 39. The arm of the bridge between apices 40 and 42 includes fixed resistor R1, the arm between apices 40 and 43 includes fixed resistor R and the arm between apices 4i and 42 includes adjustable resistor R3. Apex 43 is connected to ground G through conductor 48. The arm of the bridge between apices 4i and 43 includes variable condenser C1, conductors 49 and 49'- connecting the bridge to the insulated cable B, conductors 25 and 25' of cable 3, conductors 24 and coil 23. In other words, the arm of the bridge between apices 4| and 43 includes coil 23 within instrument A which is adapted for movement along the axis of the borehole, electrical connections for connecting this coil with the bridge 33 and a variable condenser C1 adapted to be positioned at the surface of the earth.

In the logging of a borehole, it is desirable to conduct the operation in the following manner. With instrument A, cable B and the power supply and recording means C at the surface of the earth adjacent the borehole, the frequency source 33 is adjusted until the tube II in instrument A is at resonance and the bridge 33 is then brought into balance by" suitable choice of values for R3 and C1, that is, a null reading of the tuned detector and recording galvanometer 38 is obtained. Bridge 33 is then unbalanced by alternating R3 or Cl separately or simultaneously until galvanometer 38 gives a reading at approximately the mid position of its scale. Instrument A is then lowered along the axis of the borehole by means of cable B and as it passes a through the formations penetrated by the borehole. the variations in the galvanometer readings indicate the reflection characteristics of the formations. It will be understood that galvanometer 38 may be of the recording type, as illustrated.

The direct current produced by supply 3| acts to polarize that portion of tubular member ll adjacent coil 23 and when this portion of the member is subjected to the alternating magnetic field produced by the alternating current flowing through coil 23, a vibration-is set up in tubular member ll. When tubular member II is vibrated in this manner the ends. move longitudinally and have an appreciable amount of movement while the medial portion of the cylindrical surface moves radially and has a magnitude of movement of the order of /100 the magnitude of movement of the ends. But because of the relative areas of the surface the major portion. of energy removed from the vibrating system is taken from that portion of the system vibnating in a radial direction and these radially directed vibrations pass to the formations adjacent instrument A. When the vibrating system is vibrating the amount of energy removed from and not reflected back into the system is a function of the transmission characteristics, and hence of the reflection characteristics of the formations adjacent instrument A. For maximum efficiency and sensitivity it is desirable to pass through coil 23 an alternating current of very nearly the same frequency as that of the vibrating system. Since the magnetostrictive tube H vibrates when alternating current is applied to coil 23, it is apparent that the tube l l is electromechanically coupled to coil 23. Accordingly, it will be seen that when the tube H is damped, as by reflection of energy from the adjacent formations back into the system, the apparent impedance of coil 23 is changed and bridge 33 is unbalanced. The unbalanced condition is transmitted through detector 32 and produces variations displayed in recording galvanometer 38. ployed toproduce a log which is a function of the reflection characteristics of the formatio adjacent instrument A.

By way of examplathe following dimensions These variations may be em may beemployedin the construction of the embodiment shown in Fig. 1: The tubular member I I may have a, length of the order of 9 feet, an outside diameter of 3 inches and. an inside diameter of 2 inches, and the coil 23 through which the current is passed to drive the system may have a length of approximately 1-2. inches. A vibrating system of these dimensions may be vibrated in resonance with a current having afrequency of 8'70. It will be understood that. the frequency of the system is a. function of the physicaldimensions thereof. With the tube described, resonant at a frequency or 8'70 cycles, it-is desirable to maintain the frequency of the exciting current Within one-half cycle of the resonance frequency of the tube.

It may be seen that an acoustic wave generator of the. above-described dimensions, when caused to vibrafie in resonance in the extensional mode, will-expand and contract radially and will radiate acoustic waves predominantly radially in all directions over an area whose length is in excess of 9' feet. Since the velocity of acoustic waves averages approximately 5000 feet per second or slightly less, either in water or in a typical drillin mud, it will be apparent that the wavelength of acoustic waves having a selected frequency of 870 cycles'per second will be approximately 5.7 feet in either of these media. It will also be apparent to workers inv the art that this wavelength is long; compared with the usual diameter of boreholes in which formation logs are gen erally run. Thus, when a generator of the abovedescribed dimensions is lowered into a mudfilled borehole and operated at its resonant frequency acoustic waves will be. radiated predominantly toward the walls of the borehole throughout an, area whose length is many times the diameter of the borehole. Since a generator or transducer of the above described dimensions is capable not only of emitting acoustic waves but also of receiving reflected acoustic waves, it will be apparent that the received waves will be reflected thereto predominantly from an area of the borehole walls whose length is greater than the wavelength of the waves. If it is desired to log formationsin boreholes havin drilled diameters greater than the usual 8 to 12 inches, the above-described dimensions may be increased and the resonant frequency will preferably be reduced. 7

Fig. 2 shows a view of another vibrating system which may be substituted for the vibrating system of Fig.1. The, vibrating system of Fig. 2 differs from that of Fig. 1 in employing a rod or bar 50 as the elongated member which is driven to vibrate at a resonance frequency. Rod 59 is constructed of a suitable magnetostrictive material, such as nickel, and is mounted by means of a pin passing through a hole drilled transversely through its center. It will be understood that transverse pin 5| may be secured to member [2 by any suitable means to position the vibrating rod 58 within the outer shell defined by members I2, [3 and I4 so that instrument A may be raised and lowered along the bore: of a well without exposing the vibrating system to drilling mudand foreign objects in the borehole. The driving means for rod 50' includes a coil 54' arranged: around the medial portion thereof and connected through leads 5'2 and 53 to conductors: 2'5 and. of cable B. The means for supplying power and for: obtaining a record at the surface may be exactly the same. means described and shown in Fig- 1', and accordingly a description of the power supply means and indicating means will not be. repeated- Another embodiment of the vibrating system adapted to be substituted for that of Fig. 1 is shown in Fig. 3.v In this embodiment the vibrating system includes a tubular member 60 formed of a magnetostrictive metal. The means for driving tubular member 69 includes a toroidal wound coil BI with the ends thereof connected through conductors- E2 and 63 to insulated conductors-25 and- 25 of cable B. Clamping means 64 and 55 are arranged at the ends of the coil and are. secured in turn to the end portions I3 and I 4 which secures the vibrating system in position with respect to the outer shellcomposed of members I2, I3; and I4.

In the. embodiment of Fig. 3 the supply of power to coil 6| causes at least. a major portion of tubular member 60 to move radially. It will be understood that the ends of member 60 may move a slight amount in the longitudinal direction while the. remainder moves radially. The power supply means and indicating means employed with the vibrating system of Fig. 3 may be exactly the same as that shown in the embodiment of Fig. 1.

Another embodiment of the present invention is shown in Figs. 4 and 5. In this embodiment a tube of magnetostrictive metal is driven in a manner similar to the embodiments of Figs. 1 and 2. Accordingly, this tube is designated by numeral II employed. to designate the tube in- Fig. 1. In this embodiment, tube II is arranged in an outer tubular member I2v provided with a lower end member I4. This embodiment difters from Fig. l in being provided with an upper end member l3 which defines a cavity 82 to house a portion of the electrical equipment designated generally by 83. End member I-li corresponds to end member M of fig. 1 and is provided with a passage 18 closed by a cap screw [9- and, in like manner, end member I3 is pro vided with a passage I6 closed by cap screw H. The passages in the end members allow the cavity defined by elements I2, l3 and I4 to be filled with liquid in exactly the same manner as the corresponding cavity in the apparatus of Fig. 1. In this embodiment a spool-shaped member 22 is. secured torod member In and a cross pin I5 secures a center portion of tube III to rod 10. Adjusting screws 20 and 2| are also provided to hold the central portion of vibrating tubular member II in position with respect to the shel-lincluding members l2, I3 and I4.

Theelectrical portion of the device includes coils 84 and 35 which are mounted on a spoolshaped member 22 and serve to: drive tubular member H. An indicating instrument, such as a recording galvanometer 86, and a means, such as a battery 87, for supplying direct current is arranged at the surface andconnected toinsulated conductors 25 and- 2'5 of cable B. Arranged within cavity 82 of upper end member I3 is a container 83 in. which are vacuum tubes 85% and 89, coupling condenser 90, choke 91. and resistor 2. It willbe seen that one end of coil 85 is connected to the grid of tube 89 by conductor 23 while the other end of coil 35 is con nected to conductor 25 of cable B by conductor 94. Similarly, one end of coil 84 is connected to the plate of tube 88 by conductor while the other end of coil 84 is connected to conductor 25. of. cable B by conductor 96. This system is self-oscillatory, that is, the rod II will oscillate. at its natural frequency. As the instrument Av is passed along the axis of a borehole, the reflection constant of the formations will be in- 'dicated by readings of the recording galvanometer 86. Although in the drawing an oscillator employing two electron tubes having elements connected to coils electromechanically coupled to a magnetostrictive element has been shown, it will be understood that other electron tube oscillators employing one or more vacuum tubes may readily be substituted by a skilled worker in the art.

Still another embodiment of the present invention is shown in Figs. 6 and 7. In this embodiment a tubular element made of magnetostrictive metal is caused to vibrate in a manner similar to the embodiments of Figs. 1 and 5. Hence this tubular element is again designated by the numeral H as in Figs. 1 and 5. The em bodiment shown in Figs. 6 and 7 is preferably operated in conjunction with an electrical portion similar to that shown in Fig. 4 and, accordingly, the physical structure may be compared with the embodiment shown in Fig. 5. The structure differs from that shown in Fig. principally in the mounting of the coils within the magnetostrictive element l I and the mounting of the latter within the outside shell [2.

In this embodiment, a ring wedge 66 is securely fastened to a medial portion of tubular element H. The outer shell is constructed of relatively thin walled non-magnetic tubular elements 6'! and 67 which are rigidly afiixed, as by welding, to a ring member 63. Ring member 68 isprovided with a beveled inner edge adapted to provide a seat for ring wedge 66. means, such as screws 68, are passed through member 6? at diametrically opposite positions in a manner so that they force ring wedge 66 to seat firmly in ring member 88. In this manner,

tubular element H may be mounted securely at a single plane which is transversely across the tubular member at a medial portion thereof.

As in Fig. 5, coils 84 and 85 are mounted within tubular element H. As will be seen from Fig. 7, a non-magnetic metal plate member 70 is rigidly fastened within tubular element H by a plurality of diametrically opposite pin members 7| which may conveniently pass through ring member 66. Coils 84 and 85 are wound upon spool-like elements 72 and 73 which are clamped together'on opposite sides of plate It by means of a non-magnetic metal bolt or threaded rod 74 having nuts l5 screwed on each end thereof. In this way, the entire coil structure, including plate 70 and spool-like elements 72 and 13 havi ing coils 84 and 85 wound thereon, may be positioned within tubular element II and spaced away from element l I at all points except where pin members 7| pass therethrough.

Since the remaining parts of the apparatus shown in Fig. 6 correspond with the parts shown in Fig. 5, a further description of these parts is not necessary.

While I have described several specific embodiments of the present invention, it will be evident to a worker skilled in the art that various changes may be made in the apparatus without departing from the scope of the invention.

Having fully described and illustrated the present invention, what I desire to claim is:

1. An improved apparatus for acoustic impedance logging of formations along the liquid- Suitable f.

10 filled portion of a borehole in the earth comprising, in combination, a transducer of acoustic wave energy of substantially a selected frequency having such a value that the wavelength of said wave energy in the borehole liquid is longer than the diameter of the borehole, said transducer comprising an elongated element constituting a wave enerating surface having a greater to said electromagnetic means as the transducer is moved along the formations.

2. The improvement in accordance with claim 1 in which said elongated element is a substantially cylindrical, magnetostrictive metal element which is magnetically polarized.

3. The improvement in accordance with claim' 2 in which said elongated element is a tube supported solely midway of its length.

4. In apparatus for acoustic impedance logging of formations along the liquid-filled portion of a borehole in the earth, the improvement which includes a transducer of acoustic wave energy of substantially a selected frequency having such a value that the wavelength of said wave energy in the borehole liquid is longer than the diameter of the borehole, said transducer comprising an elongated element constituting a wave generating surface having a greater length than said wavelength and electromagnetic means operatively associated with said element for causing the latter to vibrate in extensional mode at said frequency whereby said element expands and contracts in all directions transverse to its longitudinal axis and generates symmetrically expanding waves substantially throughout its length.

5. The improvement in accordance with claim 4 in which said elongated element is a substantially cylindrical, magnetostrictive metal element which is magnetically polarized.

6. The improvement in accordance with claim 5 in which said elongated element is a tube supported solely midway of its length.

CLARE H. KEAN.

REFERENCES CITED The following references are of record in the file of this patent:

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